Well over half of millennial parents say their parents provide at least an hour of child care or help running the household each week, and the average grandparent went all out, spending 48 hours per week on tasks including primary child care, babysitting, homework help and transportation to after-school activities.

The (hopefully grateful) parents estimated that the grandparents’ support came to $300 per week on average, which could add up to as much as $15,600 per year.

And the help provided wasn’t because the millennial parents were strapped either, said David Lynch, managing director and head of branches at TD Ameritrade.

“Millennials are waiting until they are financially secure to have kids, far more than their parents did,” he told CNBC.

But, he added, this is a generation burdened by student loan debt and stagnant wages. About 25 percent of millennial parents receiving hourly support and 18 percent receiving financial support say they couldn’t afford their current lifestyle without that help.

Grandparents seem more than willing to do so. Forty percent said their adult child didn’t ask for help — they were the ones to offer it — and 43 percent said they did it because “it makes me happy.”